1. Field of the Invention
The present invention relates to a lens device including a position detection unit for detecting the position of a movable portion such as an optical element or an operation portion, an optical apparatus such as a camera or a camera system to which the lens device can be attached, and an apparatus such as an operation device for the lens device. The present invention also relates to a method of setting a reference position of the position detection unit.
2. Description of the Related Art
A lens device connectable to a camera includes movable portions. Examples of the movable portions include multiple lenses driven for focusing and zooming, and a diaphragm for adjusting a light amount. The lens device further includes operation rings and demand portions which are used to drive the respective lenses.
In a normal rear-focus-type lens device, the positions of an optical element and an operation device are detected and position control is performed based on the detected positions. Therefore, in order to obtain a desirable focus state, it is necessary to improve the position detection precision of each of the optical element and the operation device.
For example, a rotary encoder may be used. According to the rotary encoder, a relative position is detected based on two signals whose phases are different from each other, that is, an A-phase signal and a B-phase signal. Further, a Z-phase signal for detecting a reference position is obtained. Therefore, relative position calculation for absolute position signal conversion is performed using the reference position as standard.
There is a case where a rotary encoder in which the Z-phase signal is not detected and instead a reference position detection unit, such as a potentiometer, is used. The operation device and the lens device which employ the position detection method are initialized in the following procedure.
In an initialization process of an operation portion of a zoom operation device (zoom demand device), a camera operator first determines that the zoom demand device is powered on and then operates the operation portion.
When the operation portion operated by the camera operator passes through a reference position of a reference position detection unit, an output of the reference position detection unit changes. A rising position or a falling position of the output is associated with a relative position indicated by a counted value for a relative position detection unit. A signal indicating a value obtained by subtracting an offset amount of the reference position (which is stored in advance) from the relative position is output as an absolute position signal.
As described, for example, in Japanese Patent Application Laid-Open No. H10-274737, in the case of a focus lens or a zoom lens in the lens device, when the lens is automatically moved after power-on, the counted value for the reference position detection unit and the reference position thereof are associated with each other and the initialization process is automatically performed.
The detection response speed of the relative position detection unit is different from the detection response speed of the reference position detection unit, so a detection position of the relative position detection unit is shifted from the reference position of the reference position detection unit.
The detection position is particularly significantly shifted in the case where the operation portion of the operation device or the optical element of the lens device, each of which is a position detection object, is moved at high speed. When the position detection precision of the operation portion of the zoom demand device is low, an accurate drive control signal corresponding to an operation amount of the zoom demand device is not output.